The selection and application of a tone scale adjustment function is a key component to properly display multiple modalities of medical images on a common display device such as a soft copy monitor.
Current medical imaging soft copy systems implement fixed tonal processing parameters for all images. As certain modalities require different tonal processing parameters to be rendered correctly, these modalities are currently displayed with sub-optimal image quality. While DICOM (Digital Imaging and Communication in Medicine) format allows for the specification of a tonal transformation for image display, the transformation is not required and the output space of the transform is not well defined. Therefore, images from different vendors may, and typically do, require different tonal processing to achieve the desired output. For hardcopy, the tone scale may be modified by a `curve shape` parameter in the printer; there is no analogous parameter for soft copy. There is a need to determine the appropriate tone scale adjustment to be applied to an image for soft copy display based upon information in the DICOM header so that the image is rendered correctly.
Muka, Blume and Daly ("Display of medical images on CRT soft-copy displays: A tutorial", Proc. SPIE, Vol. 2431, pp. 341-359, 1995.) describe differences in tone scales between the following modalities: computed tomography (CT), magnetic resonance (MR), ultrasound (US), digitized film and computed radiography (CR). Muka, et al. states that all images of a particular modality require the same tone scale application for proper cathode ray tube (CRT) visualization. While this is true for a large number of cases, this is not true in every case. For example, the CR images of one manufacturer are processed and distributed with code value linear to density, as described. CR systems of another manufacturer process and distribute images with code values linear to CRT digital to analog converter (DAC) values.
Mills, Ingram, Spetz and Dwyer ("Suitability of non-linear display tables to clinical PACS", Proc. SPIE, Vol. 2435, pp. 2-10, 1995.) report that applying a CRT gamma adjustment that the user can modify interactively on a soft copy workstation yield diagnostic improvements for CR images. Mills, et al. propose that similar results may be obtained for other modalities including MR, CT and digital fluoroscopy based upon results from CR. Planned research includes automation of gamma correction "based upon anatomical region, modality, acquisition device characteristics and image histogram data". The proposed research work described in the paper indicates additional information is required to select the tone scale, namely knowledge of anatomical region and image histogram data.
Przybylowicz, et al. (U.S. Pat. No. 5,671,070, issued Sep. 23, 1997) describe a method for sending an image across a network such that the tone of the image is properly rendered on the output device. Przybylowicz et al. requires that the acquisition device generate and send, along with the image, a LUT containing information on how to tonally render the image. Various methods for creating the final LUT to be applied to the image are described. As a soft copy device may display images from various vendors, a method for proper tonal rendering of all images, regardless of source, is desired.
Hishinuma (U.S. Pat. No. 5,260,873, issued Nov. 9, 1993) describes an apparatus for applying a gradation table or tone scale to a medical image. Hishinuma does not indicate how to select the gradation table to be applied to the image.
Beaudin et al. (U.S. Pat. No. 5,250,933, Oct. 5, 1993) describes a method and apparatus for applying a different color mapping or tone scale to different images on the same system. Beaudin et al. does not claim a method for determining what LUT should be applied to the image.